1. Field of the Invention
The present invention relates to a sacrificial electrode material consisting of an magnesium-based alloy and providing electrochemical corrosion protection to metallic articles exposed to an aqueous electrolytic solution, such as copper condensate tubes or iron tubes used in heat exchangers or the like which are exposed to sea water or other similar environments.
2. Description of the Prior Art
In general, electrochemical corrosion-prevention methods using magnesium-based alloys or zinc-based alloys as anodes have been employed for the purpose of protecting structural parts or members of heat exchangers or the like from corrosion.
Particularly, since anode materials made of magnesium or magnesium-based alloys are electrochemically base relative to the structural materials of copper alloys or iron alloys used in heat exchangers, they have been expected to be suitable as sacrificial anode materials for corrosion prevention. Despite this advantageous property, the conventional magnesium-based alloy materials have not yet been widely used as sacrificial electrode materials.
The reason for this is considered to as follows. As the magnesium-based alloy sacrificial electrode materials, Mg--Al--Zn alloys have been used, but they are useful within the content ranges of Al and Zn of less than 7 atomic % and less than 4 atomic %, respectively. When the contents of Al and Zn in the alloys exceed these content ranges, the resulting alloys have a significantly noble spontaneous electrode potential and are unsuitable for use as sacrificial electrodes.
Further, in the above Mg--Al--Zn alloy sacrificial electrode materials, transition metal elements, such as iron, nickel, copper, etc., are controlled to 30 ppm or less in their total. When these elements are present as impurities or alloying elements in the alloys, the self-corrosion resistance of the materials considerably reduces and the useful life as sacrificial electrodes becomes short.
Conventional materials obtained by casting or by subsequent rolling are composed of coarse crystal grains. Therefore, when such conventional materials are employed as sacrificial electrode materials, corrosion selectively proceeds along crystal grain boundaries, and, thereby, separation and breakage of the materials occur. Consequently, the useful life as sacrificial anodes significantly reduces. Especially, when the above-mentioned transition metal elements coexist as solute elements or impurities, the above-mentioned tendency is considerable. Therefore, the contents of the transition metal elements have been very strictly limited.
For the foregoing reason, the use of the conventional magnesium-based alloy materials as sacrificial electorodes has been limited to a narrow range, although they are electrochemically base as compared with aluminum-based alloys or zinc-based alloys.